THE CRANIAL NERVES. 455 



bral convolutions. It is often unaccompanied by any perceptible altera- 

 tion in the tissues of the eyeball or the optic ner. 



But it is not easy to account for blindness confined to the opposite 

 eye. from lesions on one side of the brain, if each optic tract contains 

 fibres destined for both eyes. We know that injury of one optic tract 

 produces hemiopia in both eyes : and the only plausible explanation of 

 this fact is in the supposed double distribution of its fibres. But it is 

 equally certain that cerebral lesions of one side, above and behind the 

 optic tracts, produce, on the contrary, blindness in the opposite eye. 

 It is supposed by Charcot that a supplementary crossing may take place 

 behind the central attachment of the optic tracts, like that indicated 

 in Figs. 121 and 122. According to this view the crossed fibres of the 

 right optic tract, which have come from the left eye, communicate with 

 the right side of the brain ; while its direct fibres, which have come 

 from the right eye, cross the median line, perhaps in the tubercula 

 quadrigemina, and communicate with the left side of the brain. Thus 

 a region somewhere in the cortex of the left hemisphere will represent 

 all the fibres coming from the right eye, and a corresponding region 

 in the right hemisphere will represent all those from the left eye. This 

 hypothesis still leaves some points of difficulty in regard to unilateral 

 blindness and hemiopia, but it affords the most rational explanation of 

 their principal phenomena. 



In birds, the reflex stimulus which causes contraction of the pupil 

 passes, owing to the complete decussation of their nerves, to the optic 

 tubercle of the opposite side. But here, by the transverse connection 

 of the parts, it becomes duplicated ; and a beam of light, falling upon 

 one retina, will produce contraction in both pupils. Even when one 

 optic tubercle has been removed and the opposite eye permanently 

 blinded, both pupils will contract under the stimulus of light falling 

 upon the sound eye. In examining an eye, therefore, either in animals 

 or in man, to determine whether it be sensitive to light, the opposite 

 eye should always be covered, in order to prevent its exciting a move- 

 ment by crossed reflex action. 



Third Pair. The Oculomotorius. 



The oculomotorius nerve, so called because it supplies most of the 

 muscles moving the eyeball, originates from a collection of gray sub- 

 stance next the median line, beneath the tubercula quadrigemina and 

 the aqueduct of Sylvius. As this group of nerve cells is continuous 

 with that which gives origin to the fourth nerve or patheticus, it is 

 designated as the common nucleus of the oculomotorius and patheticus 

 nerve,*. From this nucleus the fibres of the oculomotorius pass down- 

 ward and forward, through the crus cerebri, until they emersre, in the 

 form of several flattened bundles, from its inner border, a little in front 

 of the pons Tarolii. They then unite into a rounded cord, which runs 

 forward and outward, to penetrate the cavity of the orbit by the 

 sphenoidal fissure. During this transit the nerve receives one or two 



